The present invention relates to the general field of gas exhaust nozzles for bypass turbomachines, and more particularly to a turbomachine nozzle having an exhaust or throat section that is variable as a function of operating speed.
A bypass turbomachine nozzle typically comprises an annular central body centered on a longitudinal axis of the nozzle, an annular primary cowl coaxially surrounding the central body to co-operate therewith to define a primary annular channel, and an annular secondary cowl coaxially surrounding the primary cowl to co-operate therewith to define a secondary annular channel coaxial about the primary channel.
The term “nozzle throat section” is used to designate the cross-section of the secondary channel that is the smallest along the entire length of the nozzle. The term “nozzle exhaust section” is used to designate the cross-section of the secondary channel that is the furthest downstream.
It is known that by varying the exhaust section or the throat section of the nozzle of a turbomachine, it is possible to control the rate of flow through its fan so as to place the fan under operating conditions that correspond to optimum efficiency, at any speed of the turbomachine. The use of exhaust nozzles of geometrically variable section is thus common practice in military applications. The techniques used generally have recourse to flaps disposed to extend the downstream end of the nozzle and capable of being steered so as to reduce or increase the exhaust section or the throat section of the nozzle.
Unfortunately, those techniques are difficult to adapt to the nozzles of civilian turbomachines. This is due in particular to constraints associated with how the nacelle is installed relative to the wing of the airplane, to ground clearance, and to the thicknesses and the shapes of the trailing edges of the nacelle. In addition, such variable section nozzles are relatively expensive to fabricate.
Thus, the nozzles used in civilian applications are generally of exhaust or throat section that is geometrically fixed and optimized for cruising flight, since that represents the major fraction of the mission of an airplane. As a result, the fixed section nozzles operate suboptimally when the turbomachine is running fast (corresponding to takeoff and while the airplane is climbing), and while the turbomachine is running slowly (corresponding to descent, to the approach stage, and to the airplane idling in flight).